Conventional wheel alignment turning radius plates are usually heavy and inconvenient to move, clean, and handle. Their weight and bulkiness make them unacceptable for use in a light-weight, portable set of alignment equipment. The need for this type of set exists in service stations of limited space, in field safety test work, in private service operations, and as an accurate back-up capability for large alignment installations.
Furthermore, most existing portable equipment of this type includes turning radius plates of the semifloating type which have the distinct disadvantage of promoting errors when making caster readings. This is due to the fact that most vehicle front wheels do not turn on a pivot point under the tire when the wheel is steered or swung as when taking a caster reading. The scuff radius associated with most front wheels makes it mandatory that full-floating turn plates be used if rolling of the wheel on the plate is to be avoided while the wheel is swung. Since taking caster readings, as well as toe-out-on-turns readings, depend for their accuracy on exact swing angles (usually 20.degree. each way) from zero (straight ahead), unacceptable errors may occur if the wheel rolls on the plate thus making the operator swing the wheel farther than the specified amount which the turning scale then erroneously indicates. It can easily be shown that a one degree error in wheel swing each side of zero will produce an error of close to 5 percent for any caster reading. Thus an error of two degrees in said swing angles results in a 10 percent error in caster, and similarly a 4.degree. error in swing angle produces a 20 percent error in the caster reading.
For the above reasons, and as a result of two discoveries a new set of turning radius plates and swing angle indicators has been developed. It has been found that two thin aluminum (or other rigid material) plates when properly processed will effectively act as turn tables for the heaviest vehicles. Furthermore, these plates function effectively as full-floating due to a low-friction, interfaced material between them. It has been found that when one of said plates (preferably the lower one) is coated with tetraflouroethylene ( T F E ) in the form of resin or impregnated fibres the frictional resistance to the turning of one plate on the other is reduced to a very satisfactory level. The T F E material may be in the form of woven fabric and same may be applied to the plate by an epoxy paste mix or other similar means to achieve a permanent dry coating. Since the tow plates of a single turn table are not fastened together in any way, they may be instantly cleaned if necessary by a few wiping strokes. The T.F.E. fabric need be only 0.005 to 0.010 inch in thickness. Liquid deposition of T F E may also be use satisfactorily. It has been found that only one plate should be coated with T F E to attain minimum friction, thus providing a sliding interface between this material and bare, smooth metal. The coefficient of friction thus attained is approximately 0.02 which represents a 90 percent reduction from the coefficient of friction encountered between two bare metal plates. Thus the full-floating, load carrying requirement for such plates is obtained in an exceptionally thin device. For example, turning plates found thru tests to be fully practical are merely one-quarter of an inch in overall height, consisting of two one-eighth inch thick discs.
The thin profile of the turn tables described is also advantageous due to the small difference of one-quarter inch or less between front and rear wheel elevation when the vehicle being tested rests on a level plane. The small rise of the front wheels above the plane of the rear wheels due to the turn table thickness produces an error in caster of less than one-eighth of a degree which may be neglected. This is in contrast to other turning plates which are usually one inch or more in height thus requiring an adjustment of either the front or rear of the vehicle support plane. If such adjustment is not made when using conventional turn tables a caster error of one-half degree or greater will result.
In addition it has been discovered that errors in wheel swing angles as previously discussed herein may be fully eliminated by providing separated wheel swing angle scales. This is accomplished by also providing a moving scales indicator in the form of a horizontally supported rod attached to the wheel itself. Thus the actual number of degrees thru which the wheel is swung are also traversed by said indicator rod. Visual alignment of this horizontal rod with an angular scale line produces an accurate indication of the wheel swing angle since the scale and its angular lines act as a fixed base of reference separated from the wheel and turn table.